Single side band modulation system



Feb. 7, 1950 R. H. DlcKE SINGLE SIDE BAND MoDULATIoN SYSTEM 2 Sheets-Sheet l Filed May 23, 1945 FIGURE IA FIGURE IB FIGURE 3 ATTORNEY Feb. 7, 1950 R. H,A DlKE 2,496,521

SINGLE .SIDE BAND MODULATION SYSTEM ATTORNEY Patented Feb. 7, 1950 SINGLE SIDE BAND MODULATION SYSTEM Robert H. Dicke, Cambridge, Mass., assignor, by mesne assignments, to the United States of America as represented by the Secretary of War Application May 23, 1945, Serial No. 595,359

This invention relates to communication systems and more particularly to systems involving the use of single sideband modulation and transmission.

According to conventional theory, when energy waves of two frequencies are combined to produce an amplitude-modulated wave, the resultant signal contains components at new frequencies. 'I'hese new frequencies include the sum and difference of the two original frequencies, the signal components at these frequencies being known as sidebands. Several distinct advantages have led to the use of single sideband transmission inv which either the sum or the difference frequency component is eliminated. Increased ratio of power in the sideband to carrier power and narrower band width are important advantages. Narrower band width makes possible a higher signal-to-noise ratio and improved intermediatefrequency amplifiers in the receiver. One very great disadvantage however has been encountered. The equipment necessary for sideband elimination was extensive in both transmitter and receiver. Highly selective tuned circuits were involved, and even then it was not possible to obtain a sharp distinction between two frequencies which were very close to each other.

Many distance measuring devices for airplanes and other vehicles have been made using frequency modulation and the Doppler effect. However, because of the difculty of separating waves of two frequencies which are close together, such devices were not as desirable as could be expected.

An object of the present invention is, therefore, to provide a novel system for combining two frequencles to produce either or both single sidebands.

A further object of the present invention is to provide a system for combining either sideband with local oscillations to produce a single sideband of an intermediate-frequency signal.

Another and still further object of this invention is to provide a systemfor combining a signal containing both sidebands with local oscillations to produce either sideband of signal.

In accordance with the present invention there are provided two mixers or converters connected to the symmetrical branches of a T-bridge orA magic-'I'. The electrical distances of the two mixers from the electrical center of the magic-T differ by an odd number of eighth wavelengths. The mixers are separated at the intermediate frequency by an electrical distance of an odd number of quarter wavelengths by means of a delay electrical center of the magic-T is connected a source oi' modulating voltage. One of the asymmetrical branches oi' the magic-T connects to a local source of radio-frequency energy. This branch also contains an absorptive load. The remaining asymmetrical branch of the magic-T connects to a load.

For a better understanding of the invention, together with other and further objects thereof, reference is had to the following description, taken in connection with the accompanying drawings in which:

Fig. 1A is a perspective view showing the preferred embodiment of the magic-T in wave guide form and Fig. 1B shows schematically the electrical equivalent atvlower frequencies;

Fig. 2A is a partially schematic view of an embodiment of the invention which may be used to produce either or both sidebands as a single sideband modulator or to demodulate either sideband, while Fig. 2B shows a vector analysis of one of the modes of operation of the apparatus of Fig. 2A: and

Fig. 3 schematically illustrates another embodiment of the invention which may be used to produce or demodulate either sideband.

Referring now more particularly to Fig. 1A, there is shown a magic-T Il in wave guide form. The magic-T Il comprises branches I2, i3, I4 and I5. To form the magic-T Il, the branches I2 to I5. inclusive, have the following relationship. The axes of the four branches meet at a circuit. The mixer which is farther from the common junction. A first and a second of these branches are symmetrical with respect to a plane which passes through the axes of the third and fourth branches and which plane is also perpendicular to the electric lines of force within one of the third and fourth branches. Further, the third and fourth branches are so disposed that the electric lines of force therein are mutually perpendicular. In the preferred embodiment the axes of the first and second branches are colinear and the axes of the third and fourth branches are mutually perpendicular to each other and to the axes of the first and second branches. The exact form of T-bridge illustrated and the designations of branch numbers are for purposes of explanation only and are not to be taken as any limitation of the invention herein described. The magic-T Il also includes irises or curtains I6 and 2|. The irises I6 and 2| are so arranged at the junction of branches I2 to I5, inclusive, and are of such dimensions that when the magic-T I l is observed from any branch, the branch is terminated in its characteristic impedance if the other three branches are terminated in their characteristic impedances. Though matching is preferable, the invention being described does not require that the magic-T be matched, and there.- fore, the invention is not to be limited to such forms of magic-T. The magic-T of Fig. v1A is disclosed and claimed in the copending application of Robert H. Dicke Serial No. 581,695, led March 8, 1945.

Referring now particularly to Fig. 1B,.there is shown a magic-T 22 Vin a low-frequency circuit diagram utilizing conventional symbols. low frequency form of magic-T is equivalent in the results it obtains to the higher frequency form shown in Fig. 1A. As there shown, the magic-T 22 comprises transformers 23 and 2.4 and terminals 25, 25, 3i and 32. The dots 33 are used to indicate corresponding instantaneous polarity of the windings in transformers 23'andv24.

Referring now jointly to the operation of the magic-T II of Fig. 1A and the magic-T 22 of Fig. 1B. it can be shown that they are identical in all respects in the manner of their operation. It will, of course, be obvious to one 'skilled in the are that the frequencies for which these two magic-T's are designed are different. If energy enters branch I or at terminal 32, it will divide equally and arrive at branches I2 and I4 or at terminals and 25 in phase. If branches I2 and I4 or terminals 25 and 25 are terminated in their characteristic impedances, no energy will exist in branch I3 or at terminal 3 I. Conversely, if equal energies, in phase, enter branches I2 and I4 or terminals 25 and 26, all of this energy will appear in branch I5 or at -terminal 32 and none will This(A appear in branch I3 or at terminal 3l. If energy enters branch I3 or `at terminal 3I, it will divide equally and arrive at branches I2 and I4 or at terminals 25 and 25 in 180 phase opposition. If branches I2 and I4 or vterminals 25 and 26 are terminated in their characteristic impedance, no

energy will exist in branch I5 or at terminal 32. Conversely, if equal energies, in 180 phase opposition, enter branches I2 and I4 or termin-als 25 and 25, all of this energy will appear in branch I3 or at terminal 3i and no'ne will appear in branch I5 or at terminal 32. Further, if energy enters branch I2 or I4 or terminal 25 or 25, the

energy will divide equally and appear in branches I3 and I5 or at terminals 3| and 32 with no energy appearing in branch I4 or I2 or terminal 25 or 25. In the discussions which follow the term "magic- T will be taken to include both of the forms i shown in Figs. 1A and 1B. The choice of the type to be used in any instance will depend upon vthe frequency and therefore no limitations will be made concerning the form used. The branches I2 and I4 and terminals 25 'and 26 will be referred to as symmetrical terminals. 'I'he branches I3 and I5 and terminals 3| and 32 will be referred to as asymmetrical terminals.

Referring now more particularly to Figs. 2A and 2B, there are shown a schematic diagram of a circuit which may be used for either single sideband modulation or demodulation, and a vector analysis showing its operation in one speciiic case. The circuit comprises a magic-T 4I connected by its symmetrical terminals 42 and 43 to balanced mixers 44 and 45. The mixers 44 and 45 are capable of combining energy of two frequencies to produce energy of frequencies respectively equal to the sum and the difference of the two impressed frequencies. The balanced mixers 44 and 45 are disclosed and claimed in the copending application of Robert H. Dicke, Serial No.

mixers 44 and 45 as shown by electrical distances of Di, Dz. D: and D4. A magic-T 55 is connected by its symmetrical terminals 55 and 1| to mixer 44. One asymmetrical terminal 12 is terminated in a matched load 13 while the other 4asymmetrical terminal 14 connects to an element 15 through a delay line 15. The element 15 may be an energy source, amplifier or other type of load. The delay line 15 produces an odd number o! quarter wavelengths delay in the signal passing through it. A magic-T 3| isconnected by its symmetrical terminals 32 and 43 to the balanced mixer 45. One asymmetrical terminal 34 is terminated in a matched load 55 while the other asymmetrical terminal 35 connects to element 15. y

In describing the operation of this circuit (Fig. 2A) use will be made of the terms upper and lower sidebands. The upper sideband will be the frequency or frequencies which are the sum of the frequencies impressed on the balanced mixers 44 and 45 while the lower sideband will indicate the diiference frequency or frequencies. The apparatus of Fig. 2A may be used in such a manner as to combine two energies, e. g., a local oscillator energy and an intermediate-frequency energy, to produce either the upper or lower sideband or both. Let the electrical distances D1 and Dz differ by an odd number of quarter ywavelengths. D: being the larger, and let D: and D4 differ by an integral number of wavelengths, which integral number includes zero. Constant reference will be made to Fig. 2B to show the phase relationship of energy waves at various points in the system.

Energy from the local oscillator source 53 entersl terminal 54, divides into the left and right terminals 55 and 55 respectively, as viewed in Fig. 2A, and arrives at the mixers 44 and 45 in phase. Referring to Fig. 2B, the relative phase of the energy waves is indicated by vectors 5I and 32. The arrow indicates the direction of rotation and we indicates the angular velocity of the vectors in each case. Energy from element 1l, in this case a source of energy, passes through the magic-TIM and arrives at the mixer 45 with an instantaneous phase such as represented by vector 33 (Fig. 2B). Energy from element 15 also arrives at the mixer 44 but experiences a quarter wavelength delay or lag. The vector 34 (Fig. 2B) represents this energy. It should be noted that vectors 33 and 94 diifer in phase by 90, with vector 34 lagging. I'he arrow indicates the direction of rotation and m indicates the angular velocity of the vectors. 'I'he outputs of the mixers 44 and 45 are indicated by vector diagrams 35 and 35 (Fig. 2B). of the upper sideband (ori-wz), the angle 0 or 0' is added, in the direction of rotation, to the angular position of the vector 9| or 32. To obtain the graphical position of the lower sideband To obtain the graphical position l 5 ous phases of the energies at the magic-T 4|. As was stated above, the lelectrical distance Dz exceeds the electrical Idistance D1 by one-fourth wavelength. The effect of this difference in lengths of paths is indicated by vector diagrams |0.| and |02. It should be noted that in the two diagrams, and |02, the upper sidebands in the two cases are in phase,.while the lower sidebands in the two cases are in 180 phase opposition. As before mentioned, the properties of the magic-T are such that the upper sideband, equal energies in phase, will enter one asymmetrical terminal,

say 53, and pass on to load while the lower sideband, equal energies in 180 phase opposition, will enter the other asymmetrical terminal 52 and pass on to load 46. Either load 46 or 5| may be absorptive and the other an antennae or other device. It will be seen that sing-le sideband modulation has thus been effected without the use of tuned circuits, and separation of the two sidebands will be effected no matter how large or small the modulating frequency may be.

It can be shown that under the same conditions as above, Dz exceeding D1 by one-fourth wavelength and D3 being equal to D4, this same device (Fig. 2A) will work in the opposite direction. If a signal consisting of only the upper sideband relative to the local oscillator frequency i. e., containing only frequencies higher than the local oscillator frequency, or a signal containing both upper and lower sidebands, enters terminal 53 from the source 5|, a signal will be produced at which is the effect of only the upper sideband. Likewise, the lower sideband will be selected by the system if energy containing both upper and lower sidebands or only the lower sideband enters terminal 52 from source 40. In general it may be stated that the outlet at which a. given sideband appears, when the system is' used as a modulator, will select the same sideband when the system is used as a mixer or frequency converter. Hence, the system ailords a single sideband mixer circuit which will select either sideband at will. Itis to be noted that no matter how close together the upper and lower sideband frequencies are, this system will still differentiate between them. However, it should be noted that where both sidebands are present and it is desired' to differentiate between them, the local oscillator must be at a frequency which is the arithmetic mean of the two sideband frequencies.

It can further be shown that if Di and D2 are made equal and D4 exceeds D3 by an odd number of quarter wavelengths, the system of Fig. 2A may then be used as a single sideband modulator in the same way as described above. However, with this relation between the electrical distances Dl, Dz, D3 and D4 the outlet at which a given sideband appears, when the system is used as a modulator, selects the other sideband when the system is used as a mixer or frequency converter. A vector analysis similar to that used in a previous paragraph would show this to be true.

'I'he balanced mixers 44 and 45 are used because they give a better signal-to-noise output than the conventional type of mixer. Furthermore, mixers of this type prevent radiation oi' the local oscillator signal and loss of the received signal in the local oscillator. However, any type of mixer may be used here, and the present disclosure should be so construed. The -magic-Ts 85 and 8| are used because they minimize the effect of mismatched-mixers and aid in general to obtain a well matched system. If other types of mixers are employed, other coupling means willbc needed to connect them into the circuit.

Therefore, this invention is not limited to their use.

Referring now more particularly to Fig. 3, there 5 is shown a simplified single sideband modulator and demodulator system. This system comprises two mixers |02 and |04 connected to the symmetrica! terminals |05 and |46 of a magic-T IH.

The mixers |03 and 04 are located at electrical distances D1 and Dz, respectively, from the electrical center oi' the magic-T IH. The distance D: exceeds the distance D1 by an odd number oi' eighth wavelengths. A local oscillator source ||2 connects to an asymmetrical terminal H3 of the magic-T An elenent H4 connects to the remaining asymmetrical terminal ||5. The element ||4 may be a source of radio-frequency energy, an antennae or other load. By means of the delay line ||5 the mixers |03 and 04 are separated at the intermediate or modulating frequency by an odd number of quarter wavelengths.

The mixer |04 is connected to an element ||1 which may be a source of modulatingenergy, or

it may be an intermediate-frequency amplifier or other load.

The system of Fig. 3 may, as has been stated, be used as a single sideband modulator or as a demodulator. By a vector analysis similar to that used in describing the action of the system of Fig. 2A it is possible to showthat this is true. When used as a modulator, energy from element is fed to the mixers |03 and |04 where it is combined with energy from the local source H2. The resulting upper and lower sidebands of the energy oscillations pass back to the magic-T III. It can be shown that the relative phases of the upper and lower sideband frequencies are such that these sidebands will be separated. One of the upper or lower sidebands passes into terminal ||5 and to elementv H4 where it may be utilized. The remaining sideband passes into terminal Ill where it will normally be absorbed by a load (not shown). By use of a directional coupler (not shown) it would be possible to obtain this sideband also.

This system works equally well in the conversion of an upper or lower sideband into an intermediate frequency. In this case the element H4 becomes a. source of single or double sideband energy. The -upper or lower sideband will be selected, depending upon which would appear in terminal Illif the system were used as a modulator, and will be combined with energy from the local source I2 to produce intermediate-fre- 55 quency energy at element H1. If the other sideband is desired, the source ||2 and element I4 may be interchanged or a directional coupler (not shown) may be used in conjunction with terminal H3.

60 While there have been illustrated and described what are at present considered the preferred embodiments of the present invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein 65 without departing from the invention.

I claim:

l. An apparatus for obtaining single sideband modulation comprising first and second balanced mixer means; a first source of energy for 70 producing.r a. radio-frequency carrier; a first magic-T having two symmetrical terminals and two asymmetrical terminals, said first source being connected to one of said asymmetrical terminals of said first magic-T, said two symmet- 75 rical terminals of said first magic-T being con- 7 nected respectively to said first and second mixer means; first matched loading means connected to the remaining of said asymmetrical terminals of said first magic-T; Va second magic-T having two symmetrical terminals and'two asymmetrical terminals, oneof said two symmetrical terminals of said second magic-,T being connected toA said first mixer means; first delay means connecting the remaining of said two symmetrical terminals of said second magic-T to said second mixer means; a third magic-T having two symmetrical terminals and two asymmetrical terminals, said two symmetrical terminals of said thirdmagic-T being connected to said first mixer means; second matched loading means terminating one of 'said asymmetrical terminals of said third magic-T; a second source of energy for modulating said carrier; second delay means connecting the remaining of said asymmetrical terminals of said third magic-T to said second source; a fourth magic-T having two symmetrical terminals and two asymmetrical terminals, said two symmetrical terminals of said fourth magic-T being connected to said second mixer means, one of said asymmetrical terminals of said fourth magic-T vbeing connected to said second source; third matched loading means terminating the remaining of said asymmetrical terminals of said fourth magic-T; and fourth and fifth loading means respectively connected to said two asymmetrical terminals of said second magic-T, said fourth and fifth loading means being adapted to utilize the upper and lower sidebands of energy passed through said second magic-T in a desired manner. A

2. An apparatus for obtaining single sideband demodulation comprising first and second balanced mixer means; a first source of radio-frequency energy; a first magic-T having two symmetrical terminals and 'two' asymmetrical terminals, said first source being. connected to one of said asymmetrical terminals of said first magic-T, said two symmetrical terminals of said first magic-T being connected respectively to said nrst and second mixer means; first matched loading means connected to the remaining of saidv asymmetrical terminals of saidrfirst magic-T; a second source of radio-frequency. energy; a sec-- ond magic-T having two symmetrical terminals .and two asymmetrical terminals, one of said two symmetrical terminals of said second'magic-T being connected to said first mixer means, one of said asymmetrical terminals being connected to said second source; first delay means connecting the remaining of said two symmetrical terminals of said second magic-T to said second mixer means; second matched loading means connected to the remaining of said asymmetrical terminals of said second magic-T; a third magic-T having two symmetrical terminals and two asymmetrical terminals, said two symmetricalterminals of said third magic-T being connected to said first mixer means; third matched loading means terminating one of said asymmetrical terminals of said third magic-T; amplifier means; second delay means connecting the remaining of said asymmetrical terminals of said third magic-T to said amplifier means; a fourth magic-T having two symmetrical terminals and two asymmetrical terminals, said two symmetrical terminals of said fourth magic-T being connected to said second mixer means, one of said asymmetrical terminals of said fourth magic-T being connected to said amplifier means; and fourth matched loading means terminating the remaining of said asymmetrical terminals of said fourth magic-T.

3. An apparatus for obtaining single sideband modulation comprising first and secondv mixer means; a magic-T having two symmetrical terminals and twoy asymmetrical terminals, one of said two symmetrical terminals of said magic-T. being connected to said first mixer means; nrst delay means connecting the remaining of said two symmetrical terminals of said magic-T to said second mixer means; a first source of energy for producing a radio-frequency carrier, said first source being connected te one of said asymmetrical terminals of said magic-T; first loading means associated with said first source to absorb the undesired sideband; second loading means connected to the remaining of said asymmetrical terminals of said magic-T; a second source of energy connected to said second mixer means; and second delay means connecting said second source to said first mixer means.

4. An apparatus for obtaining single sideband demodulation comprising first and second mixer means; a magic-T having two symmetrical terminals and two asymmetrical terminals, one of said two symmetrical terminals of said magic-T being connected te said first mixer means; first delay means connecting the remaining of said two symmetrical terminals of said magic-T te said second mixer means; a first source of radiofrequency energy connected to one of said asymmetrical terminals of said magic-T; a second source of radio-frequency energy connected to the remaining of said asymmetrical terminals of said magic-T; amplifier means connected to said second mixer means; and second delay means connecting said amplifier means to said first mixer means.

5. n apparatus for obtaining single sideband modulation comprising first and second mixer means; a first source of radio-frequency energy; a first magic-T having two symmetrical terminals and two asymmetrical terminals, said first source being connected to one of said asymmetrical terminals of said first magic-T, said two symmetrical terminals of said first magic-T being respectively connected to said first and second mixer means; first matched loading means connected te the remaining of said asymmetrical terminals of said first magic-T; a second magic-T having two sym-A metrical terminals and two asymmetrical terminals, one of said two symmetrical terminals of said second magic-T being connected to said first mixer means; rst delay means connecting the remaining of said two symmetrical terminals of said second magic-T te said second mixer means; a second source of energy connected te said second mixer; second delay means connecting said first mixer means to said second source; and second and third loading means respectively connected to said two asymmetrical terminals of said second magic-T, said second and third loading means being adapted to utilize the upper and lower sideband energies in the desired manner.

6. An apparatus for obtaining single sideband demodulation comprising first and second mixer means; a first source of radio-frequency energy; a first magic-T having two symmetrical terminais and two lasymmetrical terminals, said first source being connected to one of said asymmetrical terminals of said first magic-T, said two symmetrical terminals of said first magic-T being connected respectively to said first and second mixer means; first matched loading means connected to the remaining of said asymmetrical terminals oi' said rst magic-T; a second source of radio-frequency energy; a secondmagic-T having two symmetrical terminals and two asymmetrical terminals, one of said two symmetrical terminals of said second magic-T being connected to said rst mixer means, one of said asymmetrical terminals of said second magic-T being connected to said second source; ilrst delay means connecting the remaining of said two symmetrical terminals of said second magic-T to said second mixer means; second loading means connected to the remaining of said asymmetrical terminals of said second magic-T; amplier means connected to said second mixer; and second delay means connecting said rst mixer to said amplier means.

'7. An apparatus for obtaining single sideband modulation comprising first and second balanced mixer means; a ilrst source of radio-frequency energy; a ilrst magic-T having two symmetrical terminals, and two asymmetrical terminals, said first source being connected to one oi said asymmetrical terminals of said first magic-T, one of said two symmetrical terminals of said first magic-T being connected to said first mixer means; ilrst matched loading means connected to the remaining of said asymmetrical terminals of said iirst magic-T; iirst delay means connecting said second mixer to the remaining of said symmetrical terminals of said first magic-T; a second magichaving two symmetrical terminals and two asymmetrical terminals, said two symmetrical terminals of said second magic-T being connected respectively to said ilrst and second mixer means; a second source of energy connected to said'second mixer; second delay means connecting said ilrst mixer means to said second source; and second and third loading means connected to said two asymmetrical terminals of said second magic-T, said second and third loading means being adapted to utilize the upper and lower sideband energies in the desired manner.

8. An apparatus for obtaining single sideband demodulation comprising first and second mixer means; a first source of radio-frequency energy; a iirst magic-T having two symmetrical terminais and two asymmetrical terminals. said first 10 source being connected to one of said asymmetrical terminals of said iirst magic-T, one of said two symmetrical terminals of said first magic-T being connected to said iirst mixer means; iirst matched loading means connected to the remaining of said asymmetrical terminals of said first magic-T; first delay means connecting said-second mixer to the remaining of said symmetrical terminals of said rst magic-T; a second source of radio frequency energy; a second magic-T hav ing two symmetrical terminals and two asymmetrical terminals, said two symmetrical terminais of said second magic-T being connected respectively to said rst and second mixer means; second loading means connected to the remaining of said asymmetrical terminals of said second magic-T; amplifier means connected to said second mixer; and second delay means connecting said first mixer to said amplifier means.

9. An apparatus for obtaining single sideband modulation comprising a magic-T waveguide balanced mixer having two asymmetrical branches and two symmetrical branches, a. iirst source of radio frequency energy connected to one of said asymmetrical branches, loading means connected to the other of said asymmetrical branches, a iirst mixer means located in one of said symmetrical branches at a predetermined distance from the junction of said symmetrical branches, a second mixer means located in the other of said symmetrical branches at a distance from said junction diii'ering from said predetermined distance by an eighth of a wavelength, a source of low frequency energy connected to one of said symmetrical branches, and delay means connecting said low frequency source to the other of said symmetrical branches.

ROBERT H. DICKE.

REFEBEN CES CITED The following references ille of this patent:

UNITED STATES PATENTS are of record in the Number 

